/**使显示对象变暗的遮罩 */
class MaskSprite {
	public static create(obj:egret.DisplayObjectContainer):egret.Sprite{
		let mask:egret.Sprite = new egret.Sprite()
		mask.graphics.beginFill(0x000000, 0.2)
		mask.graphics.drawRect(0,0,obj.width, obj.height)
		mask.graphics.endFill()
		obj.addChild(mask)
		mask.visible = false
		return mask 
	}
}